US2890864A - Heat exchanger - Google Patents

Description

June 16, 1959 R. c. STUTZ 2,890,864

HEAT EXCHANGER' Filed April 18, 1956 I v 4 4 Sheets-Sheet 1 ATTORNEYS R. C. STUTZ HEAT EXCHANGER Jung 16, 1959 Filed April 18, 195 6 4 Sheets-Sheet 2 Fiied April, 18. 1956 4 Sheets-Sheet 4 W m M United States Patent HEAT EXCHANGER Robert C. Stuta, Buifalo, N.Y., assignor to Niagara Blower Company, New York, N.Y., a New York corporation Application April 18, 1956, Serial No. 578,928

9 Claims. (Cl. 257-37) This invention relates to a heat exchanger and more particularly to a heat exchanger for handling large quantitles of fluid to be cooled or gases to be condensed, it being illustrated, for example, 'as embodied in a condenser adapted for refrigeration service under a heavy load. The heat exchanger is also shown as being of the type in which a cooling effect is obtained largely by the evaporation of water with which the cooling coils are drenched and which is subjected to evaporation by a stream of fresh air drawn over the cooling coils. The invention is not, however, to be limited to evaporative cooling since features of the invention can be employed where heat exchange between air and a fluid is effected by the use of an air stream alone.

With smaller heat exchangers having a capacity equivalent to not more than about 85 tons of refrigeration, it is generally more economical to supply a complete unit which is made, tested and shipped to the point of use ready to be set in place and needing only electrical, water and drain connections. It is also economical to use a number of such unit heat exchangers of different capacities in various combinations to get greater capacities. However, as the load requirements increase, the cost of manufacturing, installing and maintaining multiple groups of such unit heat exchangers renders their use progressively less economical. The present invention relates to a heavy duty heat exchanger of which the components are shipped disassembled to the place of use and erected by the owner of the plant to. be served by the heat exchanger.

It is one of the principal objects of this invention to provide a heavy duty heat exchanger in which the components, particularly the parts forming the casing, can be readily shipped to the place of use at low cost.

Another object is to provide such a heat exchanger in which the casing is made of a series of similar rectangular panels which are laterally flanged at their edges and can readily be joined together to produce the complete casing by workmen having little skill and requiring no special tools beyond those necessary to shift the heat exchange coils.

Another object is to provide such a heat exchanger in which the casing is made up of a simple arrangement of sturdy low cost panels and at the same time provides proper air movement for efiicient heat transfer.

Another object is to provide .such a heat exchanger which can be adapted for evaporative cooling by the introduction of a spray tree over the cooling coils, a sump for collecting the spray water and a pump for recirculating the spray water from the sump through the spray tree and against the cooling coils.

Another object of the invention is to provide such a heat exchanger which can be adapted for use in condensing gases, such as ammonia, at high temperature through the provision of a precooling coil for reducing the sensible heat of the gases to a temperature at which they can safely enter sprayed or wetted coils without danger of accelerating the formation of or baking a cake or crust 2 derived from the hardening ingredients of the spray water upon the wetted coils.

Another important object of the invention is to provide a heat exchanger having a frameless casing composed of panels bolted together but in which any panel can be removed for access to that part of the heat exchanger without disturbing the remaining panels of the casing.

Another object is to provide such a heat exchanger casing in which most of the panels used are identical for heat exchmgers of widely different capacities and where only four of the panels are substituted by more or larger panels to provide increase in capacity of the heat exchanger.

Other objects and advantages of the invention will be apparent from the following description and drawings in which:

Fig. 1 is a perspective view of a heat exchanger embodying the present invention.

Figs. 2, 3 and 4 are enlarged, fragmentary, sectional views taken generally on the correspondingly numbered lines of Fig. 1.

Fig. 5 isa vertical central longitudinal section therethrough.

Fig. 6 is a transverse section taken generally on line 6-6, Fig. 5.

Fig. 7 is a vertical longitudinal section taken generally on line 7-"/', Fig. 6.

Fig. 8 is an enlarged, fragmentary horizontal sectional view taken generally on line 8-8, Fig. 1, and typical of the manner of joining the various panels which constitute the frame of the heat exchanger.

Fig. 9 is an exploded view of the base for the heat exchanger.

Fig. 1G is an enlarged, fragmentary transverse vertical section taken generally on line 16-10, Fig. 5.

The heat exchanger of the present invention is shown as supported upon a sheet metal base structure indicated generally at lit) and which provides a central longitudinal sump into which spray water is drained by bottom panels which incline toward the central sump. The base as shown is particularly applicable for roof installations or the like, and for installations on the ground the user may prefer to have a concrete base which is formed to provide both the sump and the sloping sides leading to the sump.

As best shown in the exploded view, Fig. 9, of the base structure It), the sump of this base structure is shown as being in the form of a rectangular sheet metal pan A having a bottom 12, longitudinal side walls 13 which are flanged outwardly at their upper ends as indicated at 14, and end walls 15 which are flanged outwardly at their upper edges as indicated at in. This pan A is flanked by two side base structures B which function to support the casing and main coils of the heat exchanger and also function to drain the spray water into the pan A. For this purpose each side structure includes a longitudinally extending inclined drain panel 18, this drain panel sloping sideways to drain over the corresponding side wall 13 of the pan A and having a downturned flange 19' which fits against the inner face of this pan side wall 13. Each drain panel 18 also has, along its upper longitudinal edge, an upturned flange 21. A second or upper drain panel 22 is provided which is laterally inclined to drain onto theupper part of the corresponding drain panel l3 and each upper drain panel 22 is provided with a downturned flange 23 which fits over the up turned flange 21 along the upper longitudinal edge of the companion drain panel 18. Each upper drain panel 22 is formed to provide, along its upper longitudinal edge, an upwardly extending side wall 24, the upper end of this side wall 24 being flanged outwardly, as indicated at 25, and thence upwardly, as indicated at 26.

Each end of each pair of the drain panels 18, 22 is secured in any suitable manner, as by welding, to the face of an upright rectangular sheet metal plate 28. Each of these upright rectangular plates 28 extends and is preferablywelded to the'vertical end edge of the corresponding wall 24 but its opposite end extends horizontally beyond the downturned flange 19 of the corresponding lower a drain panel 18 a distance approximately half the internal width of the drain pan A.

Each of these vertical rectangular panels 28 forms one wall of a supporting box having a top horizontal wall 29 s ecuredto and extendingfrom the upper edge of each vertical wall 28 in adirection opposite from the drain panels 18, 22,. a bottom wall 30 extending from the lower edge of each rectangular wall 28 under the top wall, and

rectangular walls 31', 32 extending from the opposite ends of each vertical wall 28 and also interconnecting the corresponding horizontal edges of the top and bottom walls 29 and 30. The edge of each bottom panel 30 opposite the corresponding vertical wall 28 is formed to provide an upwardly extending L-shaped flange 34 and the corresponding edge of the top panel 29 is similarly flanged upwardly, as indicated at 35. Each vertical panel 21 is formed along its vertical edge opposite the wall 28 with an L-shaped flange 36 and the corresponding edge of the opposite panel 32 is also provided with a vertical L- shaped flange 38, the flanges 36 and 38 extending in opposed relation to each other and being suitably joined to the flanges 34 and 35 to provide a rigid box structure. For additional rigidity and strength, a vertical central partition 39 is preferably welded inside of each of the boxes provided by the panels 28, 29, 30, 31 and 32 and the edge of this partition 39 can also be formed to provide an L-shaped flange 40 which is also suitably connected, as by welding, to the flanges 34, 35.

' It will be seen that when the drain pan A is laid upon the floor and when the downturned flanges 19 of the side base sections B are arranged over the side walls 13 of this drain pan, the vertical walls 31 at opposite ends of the base structure are in face-to-face relation with each other. However, it will be noted that the pan A is disposed below the bottom walls 30 of the end boxes of the side base structures B and the pan A is of suflicient length to project outwardly beyond the side base struc- 'tures B at the rear of the heat exchanger, as best shown in Fig. 5. This outward extension of the pan A is provided with a rectangular cover plate 45, and on this plate is mounted a pair of pumps 46 which recirculate the spray water collecting in the pan A through spray trees as hereinafter described. For this purpose, the inlet 48 of each pump extends downwardly through the cover and is immersed in the body of liquid maintained in the pan A. The level of this liquid can be maintained by conventional overflow and make-up water devices (not shown).

V The casing of the heat exchanger is essentially composed of a plurality of vertical panels which, except for size and the provision of pipe and other openings, are of identical form, each of these panels being rectangular and having outwardly projecting L- shaped flanges 50, 51 along its opposite vertical edges and having similar outwardly projecting L- shaped flanges 52, 53 along its upper and lower edges, respectively. Each vertical flange 50, 51 is suitably joined, as by welding, to the companion horizontal flanges 52, 53 and it will be seen that each panel has a high degree of rigidity and load bearing strength. Where the panels are joined together in straight line relationship, as illustrated in Fig. 8, the vertical flanges 50, 51 of the two panels are brought together and joined by a suitable number of through bolts 54.- Where .the panels are joined to provide a corner, as illus- 4 trated in Fig. 3, the vertical flange 50 or 51 of one panel is brought against the flat face of the companion panel and a bolt 54 is passed through this flange and body of the two panels. A layer of sealing compound 55 can be interposed between abutting faces of the panels at each joint therebetween to render the casing leak tight for air and water.

An important feature of the invention resides in the proportions and arrangement of several vertical panels which constitute the frame The casing is composed of a lower part made of a lower tier of panels, all of which are of the same height and all of which are joined together along their vertically flanged edges 50, 51 in the manner illustrated in Figs. 3 and 8. At the front and rear ends of the casing, the lower part of the casing wall is composed of a pair of central panels C and a pair of end panels D all joined through their vertical flanges 50, 51 in straight line relationship as illustrated in Fig. 8. The lower flanges 53 of these panels C and D at both the front and rear of the casing rest upon the top walls 29 of the boxes forming the ends of the side base structures B against the upstanding flanges 35 of these top walls 29, and at suitable intervals these botom flanges 53 of the vertical front and rear panels C, D can be secured to these top walls 29 by bolts 54 as illustrated in Fig. 2.

Each side wall of the lower part of the casing is made of a pair of identical side panels E arranged in straight line relationship with their opposing vertical flanges 50, 51 bolted together as indicated in Fig. 8 and with their other vertical flanges 50, 51 bolted to the faces of the corresponding front and rear panels D in the manner illustrated in Fig. 3. The bottom flanges 53 of the side panels E rest upon the flanges 25 of the base structures B against the vertical extensions 26 of these flanges 25 as best shown in Fig. 4. These flanges 53, 25 can be secured together by bolts 54 as also illustrated in this Figure 4. While the panels E, E. at each side of the casing could be made in one panel, such large size panels would be diflicult to dip galvanize and handle, and hence it is desirable to make them in the form of two panels as shown.

The lower tier of casing panels is completed by two pairs of internal panels F, F which are identical to the panels E, E except that they are joined to the opposite vertical edges of the end panels D. Thus the two panels F of each pair are arranged in straight line relationship and their opposing vertical flanges 59, 51, as illustrated in Fig. 5, are bolted together in the manner illustrated in Fig. 8 and the flanges 50, 51 of each pair of these panels F, as illustrated in Fig. 5, are bolted to the faces of the end panels D in the manner illustrated in Fig. 3. i

The casing also includes an upper part in the form or an upper tier of panels. This upper tier is formed to provide a simple rectangular 'box structure and comprises opposite front and rear end panels G, the bottom fiange 53 of each of which rests on and is bolted to the top flanges 52 of the panels C and D at each end of the casing to form upward continuations' of the front and rear walls thereof. It is a particularly important feature that the extremities of the bottom flanges 53 of each end panel G extend over and are bolted to the corre sponding ends of the top flanges 52 of both panels D at each end of the casing, this connection'being illustrated in Fig. 10. For this purpose thehorizontal extent of the'two panels C at each end of the casing are of less horizontal extent than the panel G which they support and in consequence it will be seen that either panel C at either end of the casing can be removed for access to the interior of the casing without disturbing any of the other panels of the casing. Also, any other of the panels of the casing can be individually removcd'frorn the casing for access to any of the interior parts as hereinafter described in greaterfdetaii. p The upper tier of casing panels includes two pairs 'of side panels H. These side panels H of. each pair are arranged in straight line relationship with their opposing vertical flanges 50, 51 joined inthe manner illustrated in Fig. 8 and with the other vertical flanges 50, 51 of each pair being bolted to the faces of the panels G in the manner illustrated in Fig. 3. The lower flanges 53 of the panels H rest on and are bolted to the upper flanges 52 of the panels F, as best illustrated in Fig. 6, so that these panels F, H mutually support each other and provide adequate support for the structure in the upper tier of panels, as hereinafter described.

The casing as above described is particularly shown as housing the parts of the condenser of a compressorcondenser-expander refrigeration system and employed to condense the hot refrigerant gas, such as ammonia, into liquid form. To this end, a pair of main condensing coils, indicated generally at 60, is shown as mounted in the heat exchange coil space provided by each group of panels C, D and F. Each of these main condensing coils 60 is shown as having an upper inlet header 61 and a lower outlet header 62, these being arranged one above the other alongside the corresponding front panel D and to extend horizontally parallel with these coils. These headers are connected by a multiplicity of serpentine tubes 63, the return bends of which are arranged close to the opposite end panel D, as best shown in Fig. 7. The inlet header 61 is provided with an inlet 64 for the gas to be condensed, this extending through the adjacent panel D and likewise the outlet header 62 is provided with a condensate outlet 65 extending through the same casing panel D. Desirably, as illustrated in Fig. 7, the headers 61 and 62 are embraced by a rectangu lar supporting frame 66 which is arranged parallel with the adjacent panel D and supports the corresponding end of the main condensing coil 60 directly upon its toppanel 29 of the corresponding side portion B of the base structure. Desirably, also, as illustrated in Fig. 7, the return bends of the serpentine tube 63 can be supported by a rectangular frame 67 transversely embracing the bends of the serpentine tubes and directly supporting them upon the top plate 29 of the corresponding box structure of the corresponding base section B.

The tubes 63 of the two main cooling coils 60 are constantly flushed with Water and for this purpose outlet pipes 68 from the spray water pumps 46 extend upwardly and then laterally, as illustrated in Fig. 6, and thence through the back panels D to connect and supply water to a spray tree 69 which extends the full length of each heat exchanger coil space defined by the panels D, E and F and is arranged above the main condensing coil 60 therein. This spray tree can be of the usual form having a main longitudinal central pipe '70 and branch pipes 71, and carrying a multiplicity of downwardly directed nozzles 72 which spray against the tubes 63.

Fresh air is drawn downwardly through the openings 75 above the spray trees 69 at each side of the apparatus, thence downwardly through the chambers 76 formed by the panels D, E and F and housing the main condensing coils 60 and their spray trees 69, thence horizontally along the drain panels 22 and 18 and under the vertical openings 78 defined by the lower flanges 53 of the panels F, the vertical end plates 28 and the drain panels 18, and thence upwardly through the plenum chamber 79 provided by the panels C, P, G, H; the moist, heated air being discharged through the opening 80 at the upper ends of the panels G, H.

This movement of outside air through the casing is effected by a pair of fans 81 which are of the airplane propeller type and each driven by an electric motor 82. Each motor 82 can be mounted in any suitable manner, as by an X-brace 84, the arms of which can be bolted or otherwise secured to the opposing faces of the panels HQ Preferably, an individual fan and motor is provided for each transverse pair of these panelsH and the fan 6 rotates in a circular rim 85 formed downwardly from a horizontal panel 86. Each of these panels. is rectangular and provided with upturned rim flanges 88, the rim flanges at opposing ends of these panels abutting each other, as shown in Fig. 5, and the other three flanges of each panel being disposed against the faces of the surrounding panels G and H.

With the ammonia gas to be condensed coming to the condenser at high temperatures, it is important to precool the gas before admitting the gas to the main condensing coils 60. The purpose of such precooling is to avoid the formation of a cake or crust on the spray coil 60 from the hardening ingredients of the water. To so precool the refrigerant gas, a pair of precooling coils are suitably mounted along the outsi-des of the panels H on opposite sides of the heat exchanger. Each of these coils is shown as comprising a vertical inlet header 91 having an inlet 92 for the hot refrigerant gas and a vertical outlet header 93 provided with an outlet 94 for the precooled refrigerant gas, these headers being shown as connected by a single vertical row of tubes 95. The headers 91, 93 can be secured to the sides of the panels H in any suitable manner and the coils are enclosed by a panel 96 which eX- ten-ds between and is secured to the headers 91, 93 and can be strengthened along its top and vertical edges by a flange 98 which is L-shaped in cross section as With the other panels. The 'bottom 99 of the panel is offset inwardly to engage the bottoms of the headers 91, 93 as well as to contact the sides of the panels H. The panels H are provided with horizontal slots 100, 101, these slots registering with the bottoms of the precooling coils 90 and being arranged below the fans 81. The upper end of each casing formed by the panel 96 around each precooling coil 96 is open, as indicated at on, and hence it will be seen that the fans 81 draw air in through the openings 102 down through the chambers 103 formed by the panels 90 and thence through the openings 100, 101 into the plenum chamber 79 to be discharged by the fans 81 through the outlet 80.

The pipe connections for the ammonia is illustrated by dot-dash lines in Figs. 1 and 5 from which it will be noted that the hot ammonia in gaseous form from the compressor (not shown) enters through the line 105 to the inlet 92 of the header 91 and thence passes through the tubes 95 of the precooling coil to the header 93 and its outlet 94-. The outlets 94 can be coupled together, as indicated at 106, and jointly connected by lines 108 to the inlets 64 of the main condensing coils 60. From the inlet header 61 of these main condensing coils the gaseous ammonia passes through the serpentine tubes 63 thereof and are subjected to the cooling effect of the water evaporating on the exterior of these tubes. The ammonia gas is condensed into liquid form and leaves through the outlet header 62 and outlet 65 which again, as indicated at 103, can be coupled together to insure balance of the system.

The operation of the condenser as above described can be briefly summarized as follows:

The hot ammonia gas enters from 105 into the inlets 92 and inlet headers 91 of the precooling coils 90 and passes thence through the tubes 95 and thence. from the outletheaders 93 thereof out through the outlets 94. In so passing through the tubes 95, the hot ammonia gas is subjected to the cooling effect of the streams of outside air drawn through the inlets 102 and through the chamhers 103 past the tubes 95 and thence through the openings litltl, 191 into the plenum chamber 79 from which the air is exhausted upwardly by the fans 81.

The precooled ammonia gas. from the header outlets 94 passes through the lines 106 and 108 into the inlets 64 of the inlet headers 61 of the two main condensing coils 6th. The precooled ammonia gas passes through the serpentine tubes 63 of these coils and leaves, as a liquid, through the outlet headers 62, outlets 65 and line 109.

In passing through the tubes 63 of the two main condensing coils 60, the gas is cooled and condensed by the combined efiect of the air drawn past the exterior of these tubes and the water with which these tubes are drenched, the cooling and condensing of the gas being principally a function of the evaporation into the passing air stream of the water with which these tubes are sprayed or drenched.

The air for this purpose is fresh or outside air drawn in the openings 75 and passing down through the chambers 76 and past the coils 60 so as to contribute the cooling effect of the air, both directly and through the evaporation'of water on these coils, to cooling and condensing the ammonia gas passing through these coils. This air then passes horizontally along the bottom drain panels 22 and'18 and through the openings 78 into the plenum chamber 79. From this plenum chamber the air is exhausted upwardly by the fans 81 and is discharged back into the outside air as heated air moistened by the water which has evaporated from the spray water.

This spray water is maintained in the pan A from which it is withdrawn and recirculated through the inlets 48 of a pair of spray water pumps 46, this spray water discharging through the lines 68 into the overhead spray trees 69 which are disposed above the main condensing coils 60. The water from the spray trees is discharged downwardly through a multiplicity of nozzles 72 to thoroughly wash or drench all the tubes 63 of each coil 60 and to completely cover these tubes with a constantly replenished layer of water. A part of this water is evaporated into the passing air stream both from the spray and also from the surface of the tubes 63. The water falling from the tubes 63 falls upon the drain panels 22, 18 from which the water drains back into the sump or pan A. The usual make-up and overflow devices (not shown) are provided to maintain the correct level of water in the pan A and to insure the waste of water from the system at a sufficient rate to avoid building up too high a concentration of water hardening ingredients.

It.will be particularly noted that the heat exchanger of the present invention has a casing composed of similar edge flanged sheet metal panels which can be readily joined together by the bolts 54 at the site, along with which the coils and fans can be placed and the piping connections made. It will be noted that the panel casing structure is such as to provide, with similar panels, the inlets 75, main condensing coil chambers 76 through which the air flows downwardly, the bottom openings 78 through which the air flows upwardly, the plenum chamber 79 of substantial height to enable a man to walk through this plenum chamber and obtain access to the fans 81 or other parts rendered accessible by the removal of the panels F and these panels F also provide a mounting for the precooling coils 90 and permit of using simple openings 100, 101 to draw the air past the precooling coils 90 to the plenum chamber 79.

Further a most important feature of the heat exchanger forming the subject of the present invention is the ready accessibility to all parts of the heat exchanger by removing any one of the panels independently of removing any of the remaining panels. Thus, to gain access to the plenum chamber 79, all that is necessary is to remove one of the panels C. Within the plenum chamber full access is bad to the fans 81 and also access to the opposing sides of either of the main condensing coils 60 can be had by removing any one of the four panels F. This permits painting these panels and all other internal parts of the plenum chamber as well as painting the drain panels 18 and 22 when required and the sump or pan A.

It will be noted that the removal of any panel C does' not interfere with the overhead panel G because this panel G is supported at its ends on the panels D as well as on the panels C. These panels D can also be-removed without disturbing any of the other panels since the panels G are also supported by the panels C and the'panels H and F are supported from the panels D. In other words, removing any panel D, while-re-" quiring disconnection from the end of the adjacent panel F, does not interfere with this panel B in anyway because it is supported from the overhead panel H which is in turn secured to the adjacent panel G and which is in turn reliably mounted on the panels C and the other panel D. Similarly any panel E can be' removed for access to the corresponding main condensing coil 60 and, following dismounting of either precooling coil and the corresponding fan 81, either of the panels H can be removed although there is little reason for removing these top panels, or the top, front or rear panels G, except for replacement.

Another feature of the casing is its adaptability to diflerent capacities with little alteration of the panel structure. Thus, the casing can be composed of the same panels throughout except that four sizes of the panels D can be provided for, say, 90, ton condensers, these to accommodate correspondingly larger main cooling coils 60, and two 90 ton panels D can be used to provide a ton condenser and two 120 ton panels D can be used to provide a 240 ton condenser. The other panels C, E, F, G and H can remain the same and the precooling coil 90 can have suflicient excess capacity to serve all the above sizes of condensers.

It will accordingly be seen that the present invention provides a very simple and low cost heavy duty heat exchanger which is particularly characterized by any part being very readily accessible by removing a corresponding panel from the casing which forms the housing for the heat exchanger.

I claim:

1. A frameless heavy duty heat exchanger of the character described, comprising a plurality of vertical rectangular one-piece sheet metal panels forming substantially the sole structural members constituting the side and interior walls of the enclosing and supporting casing and each panel having laterally flanged edges, means extending through said flanges for securing said panels together to join said panels in abutting relationship with one another to form said side walls, a group of at least three of said panels being joined in straight line relationship to form the lower part of the front wall of said casing, a similar group of at least three of said panels being joined in straight line relationship to form the lower part of the rear wall of said casing, additional of said panels connecting the opposite ends of said lower parts of said front and rear walls to form the lower parts of side walls for said casing, an additional pair of said panels arranged internally within the lower part of said casing in generally parallel spaced relation to each other and to said lower parts of said side walls of said casing and joined to corresponding flanged vertical edges of the panels of said lower parts of the front and rear walls of said casing to form between them a plenum chamber and between each of said pair of panels and said lower parts of the side walls of'said casing a chamber for a main heat exchange coil, a main heat exchange coil arranged in each of said chambers and extending from said lower part of said front wall to said lower part of said rear wall, each chamber for said main heat exchange co-il open at its top and there being a space under the corresponding one of said additional pair of said panels providing communication between each of said chambers for said main heat exchange coils and said plenum chamber, other of said panels mounted on the central parts of said lower parts of said front and rear walls and forming upward continuations thereof terminating adjacent the ends of said pair of panels in said lower part of the casing, other of said panels connecting the opposite ends of said upward continuations of said lower parts of said front and rear walls lower parts of said front and rear walls anupward con tinuation of said plenum chamber open at its top, and a fan in said upward continuation of said plenum chamher and supported by the panels forming said upward continuation of said plenum chamber, said fan drawing air from said open tops of said chambers for said main heat exchange coils downwardly through said chambers for said main heat exchange coils, thence horizontally through said spaces under said pair of said panels, thence upwardly through said plenum chamber, and thence out through the open top of said plenum chamber.

2. A heavy duty heat exchanger as set forth in claim 1 wherein the panels forming said lower part of at least one of said front and rear Walls of said casing are of uniform height with said upward continuation thereof extending beyond over the central panels thereof whereby one of said central panels can be removed for access to the plenum chamber without disturbing the upward continuation over said removed panel.

3. A heavy duty heat exchanger as set forth in claim 1 wherein said panels forming said lower part of said front and rear Walls of said casing are of uniform height and wherein said panels forming the upward continuations of said front and rear walls extend over the endmost panels of the lower parts of said front and rear walls whereby the central panels of said lower parts of the front and rear walls of said casing can be removed for access to said plenum chamber without disturbing said upward continuations of said front and rear walls of said casing.

4. A heavy duty heat exchanger as set forth in claim 1 wherein a separate base structure is provided for said casing, said base structure including front and rear walls respectively arranged directly below and supporting said front and rear walls of the lower part of said casing.

5. A heavy duty heat exchanger as set forth in claim 4 wherein said main heat exchange coils rest on said front and rear walls of said base structure.

6. A heavy duty heat exchanger as set forth in claim 4 wherein said base structure includes a sump under said plenum chamber and inclined sides under said chambers for said heat exchange coils draining into said sump.

7. A heavy duty heat exchanger as set forth in claim 4 wherein said base structure includes a sump into which water falling from said plenum chamber on said chambers for said main heat exchange coils drains, wherein a spray tree is arranged in the upper part of each of said chambers for said main heat exchange coils to discharge downwardly and drench the main heat exchange coil therein, and wherein a spray water pump recirculates the spray water from said sump to said spray trees.

8. A heavy duty heat exchanger as set forth in claim 7 wherein an opening is provided below said fan in at least one of said panels connecting the opposite ends of said upward continuations of said lower parts of said front and rear walls of the casing to admit a secondary stream of air to said plenum chamber, and wherein a precooling coil is arranged in the path of said secondary stream of air.

9. A heavy duty heat exchanger as set forth in claim 8 wherein said precooling coil is arranged externally of said casing.

References Cited in the file of this patent UNITED STATES PATENTS 2,502,581 Morrison Apr. 4, 1950 2,661,933 Deverall Dec. 8, 1953 2,662,741 Boyle Dec. 15, 1953 FOREIGN PATENTS 229,577 Great Britain Feb. 26, 1925 552,057 Great Britain Mar. 22, 1943

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